单项选择题

TEXT B
For hundreds of years, farmers have selected and bred plants and animals to favour, or bring out, characteristics they desired.. For example, cows that produced large amounts of milk were selected for breeding, while poor milk producers were not allowed to reproduce. Similarly, horses were bred for speed and strength. Those having these desired characteristics were selected for breeding. Over time, these preferred breeds became more common than earlier, less desired types. This selective breeding is called artificial selection.
The theory of evolution by natural selection was put forward in a joint presentation of the views of Charles Darwin and Alfred Russell Wallace before the Linnaean Society of London in 1858. Darwin and Wallace were not the first to suggest that evolution occurred, but their names are linked with the idea of evolution because they proposed the theory of natural selection as the mechanism by Which evolution occurs. We are always more likely to believe in a process when people explain how it happens than if they merely assert that it does.
The theory of evolution by means of natural selection is based on three observations. First, as we can see by comparing one cat or human being with another, the members of a species differ from one another; that is, there is variation among individuals of the same species. Second, some of the differences between individuals are inherited. (Other differences are not inherited, but are caused by different environments. For instance, two plants with identical genes may grow to different sizes if one of them is planted in poor soil.) Third, more organisms are born than live to grow up and reproduce: many organisms die as embryos or seeds, as saplings, nestlings, or larvae.
The logical conclusion from these three observations is that certain genetic characteristics of an organism will increase its chances of living to grow up and reproduce over the chances of organisms with other characteristics. To take an extreme example, if you have inherited a severe genetic disease of the liver, you have a much lower chance of living to grow up and reproduce than someone born without this disease.
Inherited characteristics that improve an organism’s chances of living and reproducing will be more common in the next generation and those that decrease its chances of reproducing will be less common. Various genes or combinations of genes will be naturally selected from one generation to the next (that is, to cause evolution). It is not necessary that all genes affect survival and reproduction; the same result occurs if just some genes make an individual more likely to grow up and reproduce.
To summarize:
1. Individuals in a population vary in each generation.
2. Some of these variations are genetic.
3. More individuals are produced than live to grow up and reproduce.
4. Individuals with some genes are more likely to survive and reproduce than those with other genes.
Conclusion: From the above four premises it follows that those genetic traits that make their owners more likely to grow up and reproduce will become increasingly common in the population from one generation to the next.
The main difference between natural and artificial selection is that human beings______.